Electric heater



De@ 31 1940- o. R. scHuRlG 2,227,159

ELECTRIC HEATER Filed March 1, 1940 JDM/MMM@ H i5 Attor-heg.

Patented Dec. 3l, 1940 'om n. schurk, Schenectady, N. Y., mmm m General Electric Company, a corporation of New York Application March 1, 194i), Serial No. 321,711

7 claims. (ol. 291-64) This invention relates isoelectric heaters and more particularly to electric heaters of the metallically sheathed type andj has for'its object the provision of an improved terminal structure for heaters of this character which is mechanically strong and fluid-tight, which is capable of withstanding high impulse voltages, and which is relatively simple and inexpensive to manufac- 1 ture and assemble.

Although this invention has other applications,

it is particularly useful in connection with e1ec' tricimmersion heaters oi the sheathed heating unit type. One immersion heater of this type is described and claimed in United States Patent 2,155,239 to C. P. Randolph dated April 18, 1939. As there described, the sheathed heating unit comprises a resistance element which is em-y bedded in a mass of heat refractory electrically insulating material and is enclosed in an outer metallic protective casing. The heating unit assembly is formed in the shape .oi a hairpin so that the terminal ends thereof areadjacent each other and these terminal endsare embedded in a large mass of homogeneous sealing and insulating material which forms a. terminal block for the terminal ends of the unit. Some difllculty has been experienced with the lterminal construction or some of these heating units due to the formation of voids or gas pockets at the Junction. between the mass of sealing material and the insulating material within the protective sheath.

Theee voids or gas pockets produce a poor bond between the two materials which manifests itself in electrical breakdown of the unit when .it is subjected to high impulse voltages such as occur during severe electrical storms.

It is, therefore, a speciilcobject of thisl invention to provide an' improved terminal structure for electric immersion heaters in which an electrically and mechanically strong bond is formed between the insulating material within the sheath of the unit and the material from which the terminal block of the unit is formed so that voids or gas pockets at the junction between these materials are substantially eliminated and the insulation resistance and dielectric strength of the terminal construction is substantially in-v creased so that electrical breakdown under high impulse voltages will be'substantially eliminated.

In accordance with this invention in one form thereof, a layer of resinous material having high insulation resistance and high dielectric strength and having the additional property of bonding to both the insulating material used within the sheath of the immersion heater and the sealing material embedding the terminal ends of the heater is placed in cup-shaped recesses formed at each terminal end of the hairpin-shaped heating unit. By providing suchv` a layer oi resinous material, voids or gas pockets at' the junction be- 5 l tween the terminal block and the insulating material of the unit are eliminated and the insulation resistance and dielectric strength of this joint are increased.

For a more complete understanding of this '10 invention, reference should be had to the ac-' companying drawing in which Fig. 1 is a fragmentary elevation of an electric heater embodying this invention, portions being broken away and portions being shown in section so as to illus- `15 trate certain details oi the construction; Fig. 2 is an end elevation of the electric heater in Fig. 1;

- and Fig. 3v is a, view similar to Fig. l showing a modified form 'of this invention.

Referring to the'drawing, I have shown my ini0' vention in-one form in connection with an electric immersion heater which is provided with a pair of sheathed heating units i0 mounted. in a suitable supporting member il. The heating units I0 have substantially the same construction and each heater, as shown, is of the helical coil sheath type' described and claimed in United States Patent to C. C. Abbott 1,36*?,34L dated February 1, 1921. Briefly, each heater comprises an outer metallic sheath i2 in which ahelical 3 electrical resistance conductor i3 is embedded in a compact mass of powdered heat refractory electrically insulating material I4,.such as magnesium oxide. The resistance conductor il has its ends secured in any suitable manner to electrical 35 terminals l5 which project from the ends ci' the sheath and which have their ends, as shown, embedded in the insulating mass il. Such units can be bent easily into any desired shape. In the immersion heater shown each heating unit is 40 bent substantially in the form of a-hairpin.

Clearly, various other arrangements and coniigurations of the heating unit may befused.

The ends of each hairpin heating unit i! are -directed through suitable spaced apertures I8 4l provided in the wall of the dished portion il of metallic supporting member i1 and are se-v cured in position by a welded joint Il. As shown in Fig. 1, the ends of the heating units' pass through from one side to the other of the wall W or the like,

shown to ce in the form of nuts threaded .onto

the terminals. As shown in Fig. 2, the correspending 'terminals oi the heaters Il are electrically connected by means of terminal jumpers or strips 2D while the pair oi opposite terminals` is electrically connected to a source of electrical supply (not shown) by means oi' leads 2l. Relatively large nuts 22 are threaded on the ends of these terminals to secure the leads therein while relatively small nuts 23 secure the Jumpers to the opposite pair of. terminals.

As described in the aforementioned Randolph patent, the projecting ends of the heater units and the terminals projecting from these ends are embedded in a single, substantially homogeneous, electrically insulating, disk-shaped mass oi sealing material 24 which is preferably formed from a suitable phenol aldehyde condensationL product. As shown in Fig. 1, the lower ends of the terminal members 20 are knurled and these knurled portions are also embedded in the insulating mass 24 so as to secure the members 28 rigidly to the terminals and to insulate them from each other.

After the heating units in have been formed, a portion ci' the insulating material il is out out of each oi the terminal ends of the heating unit so as to form a cup-shaped recess in each terminal end. In terminal structures of the type described in the aforementioned Randolph patent. this recess 25 was filled by a portion of the insulating mass 24 so that the end of the insulating material i4 was completely covered. Some dimculty has been experienced with some o! the units having such a terminal construction due to the existence oi a poor bond between the phenol aldehyde sealing material 24 and the magnesium oxide insulating material. This poor bond' was produced in part by the formation ci voids or gas pockets at the junction between the two materials and existence caused a low resistance leakage path to be formed at this junction. It was found that the presence of this low resistance leakage path aected the dielectric strength of the terminal structure so that the high impulse voltages to which the units may be subjected flur-` ing severe electrical storms caused electrical breakdowns to occur at this junction.

order to overcome this difficulty, there is provided in the recess 35 a layer 2B of resinous material which is a good insulator and which bonds readily 'to both the phenol aldehyde molding material Ehi and to the magnesium oxide insulating material il. Materials which have been found to he mrticularly suitable for this purpose are synthetic resins such as polyvinyl acetate resins produced by polymerizing pure vinyl acetate, o: thermoplastic alkyd resins produced by reacta polybasic acid in a polyhydric alcohol and, i1' desired, containing modiiiers such as oil, rosin Each of these materials. has the requisite dielectric insulation properties and, in addition. they can be applied in the molten state without excessive decompositionor gasification.

Furthermore, has been found that such materias readily adhere to both magnesium oxide and the phenol aldehyde molding composition, While these terials have been found suitable, any

resinous materials having these same propertiles might be used.

Because it more readily lends itself to manufactoring processes, it is preferred to use the polyvinyl acetate synthetic resin and in particular the polyvinyl acetate resin known as Gelva l5 which has a softening point o! about 131 C. and a molar solution oi which in bensol a viscosity oi 15 centipoises at 20 C. This particular material is solid at room temperature and may be formed in the form ci washers and placed in the recess 25. The application oi heat thereto causes the material to soften and bond to both the phenol aldehyde sealing material and to the magnium oxide.

In manufacturing the heater element described above, the heating units will first be constructed in the Iorm of hairpins and the ends of the sheath will be reamed out to provide the recesses 25. The sheath ends will then be projected 'through the apertures i8 provided for them in the support li welded to the support at Il. The units are then placed in a drying or baking oven and, after they have been baked sulciently long, they are removed and the washer or disk 28 o1' polyvinyl acetate material is inserted over the ends ot the terminal Il and pressed against the insulating material I4. The terminal members 2l and 1I are then threaded on the terminals Ii and the mass-oi insulating material 2l is applied. This material may be applied in any suitable fashion but preferably it will be applied automatically in a simple molding machine. Since the heating unitsare hot when the disk 28 of sealing material is applied thereto, this material softens and the pressure applied during the molding o! the insulating material 1 4 forces the still soit bonding material 2i to make a good bond between the phenol aldehyde molding material and the magnesium oxide insulating material within the sheath. The unit is then allowed to cool and the molding material and sealing material sets to form a mechanically strong terminal block arrangement. The operating temperature oi the terminal structure when the heating unit is used is insufdcient to cause any change in the terminal block or the sealing bond 2l and hence a terminal construction is provided having high dielectric and insulation resistance properties and having the mechanical properties essential for a water heater construction.

In Fig.. 3 I have shown a modified form ci my invention. Like numerals have been used to indicate parts already described in connection with Figs. 1 and 2. The modification oi Fig. 3 relates to the particular manner in which the resinous bonding material is applied to the terminal end. In this form oi my invention the sheath l2 oi the heater unit is flared outwardly a slight amount and the insulating material l'h is out back as in Fig. l to iorm a flared recess 21. In constructing this unit the procedure outlined above is followed and a thin washer 2i of resinous material, preferablypolyvinyl acetate resin, is placed in position over the terminal stud Il. There is then mounted over the terminal stud a lava bushing 2l which has a dared portion extending downwardly into the narod portion 21 ot the heater unit and has a head portion which extends outwardly from the end o! the heater unit sheath. The terminal screw II is then threaded onto the stud il so as to press the lava bushing rmly into position in the narod portion 21 o1 the heater unit and since the washer Il of resinous material and the lava bushing I! are applied when the unit is hot. u previously described. the washer 2O will soften and will be pressed into position so as to iorm s tight bond between the T0 lava bushing and the insulating material Il o! the heater unit. Theentirc end ofthe unit is then provided with the molded insulating block Y Il of phenol material as described above.

Prom the foregoing. it Vwill be observed that there is provided in accordance with this invention a simple and relatively inexpensive arrangement for obtaining a terminal construction for an immersion heater unit which is mechanically strong'and has improved electrical properties s0 that it is capable of withstanding high impulse voltages without breakdowns.

While I have illustrated two forms of my inventidn, it is clear that ,other modifications could be made therein and I, therefore, intend to cover in the appended claims all such modifications as are within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A terminal` construction for a sheath type heating unit having a metallic sheath and a-resistance unit located within said sheath and embedded in a compact mass of electrically insulating heat conducting material comprising a terminal block of substantially homogeneous electrical insulating sealing vmaterial molded over the terminal ends of said heating unit, and a layer or resinous material within said sheath adjacent the terminal ends thereof, said resinous material having high dielectric strength and the property oi.' readily bonding to said terminal block material and to said insulating material within said sheath so as to form a Joint between said terminal block and said insulating material capable of withstanding high limpulse voltages.

Y 2. A terminal construction for a sheath type heating unit having a sheath, a resistance unit in phenol aldehyde insulatingv material molded to,

the terminal endsoi said heating unit, and a bonding layer of resinous material within said sheath adjacent the terminal ends thereof. said resinous material having high dielectric strength and the additional property of ilrmly adhering to said phenol aldehyde terminal block and to said magnesium oxide within said sheath so as to form a joint between the magnesium oxide and the phenol aldehyde block capable oi withstanding high impulse voltages.

3. A terminal construction for a heating unit oi the sheath type having a resistance unit lccated within a metallic sheath and embedded in a compact mass ot magnesiumoxide comprising a terminal block oi' phenol aldehyde molding material molded over the terminal ends of said heater unit, and a bonding layer ot polyvinyl acetate resin within said sheath adjacent the terminal ends thereof, seid bonding layer nrmly adhering to said Y magnesium oxide and said phenol aldehyde molding composition so es to form a joint between said oxide and, said composition which is capable o! withstanding high impulse voltages.

4. An electric heater comprising a resistance conductor, a metallic sheath encasing seid oonduetor. terminals within the end portions o! said sheath connected with said resistance conductor and proiecting i'rcm said ends, a masso! electricslly insulating heat conducting material embedding said resistance conductor, and theinner endsctseidterminalnsmsssoielectricallyinsulsting sealing material molded over seid ends ot said sheath and embedding said terminals. and means comprising a resinous material interposed between said insulating materislwithin said sheath and said moldingvmaterial so as to form a joint between said materials which is capable of withstanding high impulse voltages.

5. An electric heater comprising a resistance conductor, a metallic sheath encasing said con- 5 ductor, terminals Within the end portion of said sheath connected with said resistance conductor and projecting from said ends, a mass of electrically insulating heat conducting material embedding said resistance conductor and the inner 10 ends of said terminals, bushings of electrically insulating .material mounted on the projecting ends of said terminals, a mass -oi substantially homogeneous electrically insulating and sealing material molded over said ends of said sheath 15 and embedding said bushings and said terminals, and a layer of resinous material interposed between said bushings and said insulating material within said-sheath so as to iorm a junction of high dielectric strength between said bushing and said insulating material'.

6. An electric heater comprising a resistance conductor, a metallic sheath encasing said conductor having its ends adjacent eachl other, terminals within the end portion of said. sheath connected with said resistance conductor and projecting from said ends, a mass oi electrically insulating heat conducting material embedding said resistance conductor and the inner ends of said terminals, said mass extending not quite to 30 the ends oi said sheath, so as to leave recesses in 'said ends, a mass of substantially homogeneous electrically insulating sealing material molded so as to embed the projecting ends of said sheath and the projecting 'ends o!A said terminals, and a bonding layer or resinous material mountedonsaidterminslsandlyinginsaidrecesses. said material substantially filling said recesses and bonding to said sealing material and to seid embedding material in said heater so as to form a joint between said materials having high insulation resistance and high dielectric strength so that a terminal construction is provided which is capable oi' withstanding high impulse voltages.

v7. An. electric heater comprising a resistance conductor, a metallic sheath encssing said conductor and having its ends adJacent each other. terminals within the end portion of said sheath connected with said resistance conductor me 5 projecting from ssid ends, s mass of electrically insulating -heat conducting material embedding said resistance conductor and the inner' ends of' said terminals, said mass extending not quite to the ends o! said sheath so ss to leave recesses 'f5 in said ends, s. metallic support having spaced apertures extending therethrough through which the ends of said sheath are passed i'rom4 one sidetotheotherofsaidsupportsossto project beyond the letter side ci seid wm l.. mass ot phenol sldehyde sealing material molded on said one side and projecting therefrom to embed-the miecting ends ot seid sheath and the projecting ends ot said terminals, and a bondu lng layer ci' polyvinyl acetate resinous materiel mountedonsaidtenninsisendlynginsaidref cessesisaid material sulmtsntiallyfillingsaldre-` c'ess and bonding toseid sealing material and tosaidembeddingmsterielinseidhestersoss torormajointtherebetweenwhichiscapsbie of withstanding high impuso vnltsges.

' 'OTTO R. BCEURIG. 

